This invention relates generally to balanced to unbalanced transformers (baluns) and more particularly to baluns adapted for printed circuit fabrication.
As is known in the art, baluns are used for coupling between a balanced transmission line and an unbalanced transmission line. For example, in unbalanced transmission lines current following a line, or trace, is delivered from a source to a load and then returns from the load to the source through a ground plane. The ground plane is a relatively wide conductor having relatively low resistance and inductance and thus can be shared between many circuits or applications.
A balanced transmission line, on the other hand, has two identical lines, or traces. The signals on the lines are 180 degrees out-of-phase with each other. The ground plane is used as a potential reference for the two out-of-phase signals. The ground plane has not disappeared, however, because it is used for shielding and for attachment to a chassis, for example. However, with a balanced transmission line the current flows out-of-phase through the two identical lines, or conductors, and not through any adjacent ground plane. In essence, the two out-of-phase signals are balanced over the ground plane because they draw the same amount of current in the exact opposite direction. This two-phase (i.e., balanced system) is used to introduce certain signal cancellations. Proper balance will cancel unwanted signals (e.g., harmonics or intermodulation products) and add desired ones. For example, in communication systems, a major contributor to system non-linearity is the front end mixer. To improve linearity, double balanced mixers are often used. These double balanced mixers cancel most non-linear products produced by the mixer. Because the transmission lines which precede and follow the double balanced mixer are typically unbalanced, baluns are typically required.
As is also known in the art, there are two general categories of baluns: active and passive. With an active balun a differential pair of transistors with sufficient common mode rejection may be used. If one input is AC grounded, the input voltage is split exactly in half and the two output currents are produced in opposite phase (i.e., 180 degrees out-of-phase with each other). A common base-common emitter configuration can also be used to provide proper phase split and, under the right conditions, good amplitude balance. However, in many applications, the noise and distortion levels associated with these active baluns limit dynamic range.
Passive baluns typically use magnetic coupling. They can be purchased and used with integrated circuit mixers as an external component. However, the cost of such a component is relatively high. While a passive balun has been integrated on a semiconductor chip using spiral inductors, losses tend to be relatively high because thin metal traces are used to form the inductors. Further, the area required on the chip to form such inductors is relatively large. One type of passive balun uses a printed circuit board as a substrate. This type of balun uses half wavelength delay lines that can provide the 180 degree phase shift. However, the resulting baluns are extremely large, even when operating at frequencies of 2 GHz.
Another type of printed circuit balun 9 is shown in FIG. 1A.